Most types of computing systems require memory, e.g. an element with the ability to store data or instructions. In some considerations, memory can be divided into two broad categories: volatile and non-volatile memory. Volatile memory requires a power supply to maintain the stored information. Non-volatile memory does not require a power supply and will store the information in whatever states the memory was last set to. Non-volatile memory is, however, generally subject to some form of degradation over time. Non-volatile memory is fundamental in most computing systems and includes many families of memories including but not limited to electrically erasable programmable read-only memory (EEPROM), NOR flash memory, NAND flash memory, phase change memory (PCM), hard drive memory, and even compact disc read-only memory (CD-ROM). Of these, PCM is an emerging memory option for computing systems that offers numerous advantages as technology progresses.
Phase change memory (PCM) is a type of non-volatile memory that uses a phase change in the storage material to store information. Many types of materials may be used and research is ongoing to discover and improve phase change materials, but PCM often uses chalcogenides for storage materials. Chalcogenides are a class of materials that include alloys having an element from group 16 in the periodic table (oxygen, sulfur, selenium, tellurium, polonium). A particular chalcogenide that is often used for PCM is the germanium, antimony, and tellurium alloy Ge2Sb2Te5 (more commonly referred to as GST), but many other alloys are possible.
Generally, PCM uses a storage material that can change between a more resistive phase and a more conductive phase. For example, many chalcogenides have a crystalline phase and an amorphous phase. The crystalline phase may have a low resistance and the amorphous phase may have a high resistance. The phase can be changed between amorphous and crystalline by applying an appropriate temperature. Applying a temperature above a crystallization point will cause the chemical structure to become crystalline and applying an even higher temperature above the melting point will cause the chemical structure to become amorphous.
Based on the properties of such materials, memory cells are formed of a storage material such as a chalcogenide between two electrodes. During a read operation, the resistivity will determine the current level passing through the memory cell. Thus a read operation will indicate the phase and therefore the memory state by producing a small or a large current. During a write operation, the two electrodes may apply a first current that may heat the storage material and cause it to crystallize or a second current that may heat the storage material even more and cause it to become amorphous.